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Journal of Applied Physics

Integrated scheme for transferring quantum entanglement via backward acousto-optic polarization conversion in a thin-film lithium niobate resonator

A. A. Nevzorov, K. Padalka, K. Guseva, Evelina E. Maksumova, I. Shmatkov, И. О. Венедиктов, Vadim V. Kovalyuk, Gregory N Gol'tsman

Published Oct 15, 2026• 0 citations
Publisher

Abstract

We present a numerical study of the design of a microwave-to-optical quantum converter based on Brillouin backscattering in a single thin-film lithium niobate racetrack resonator. The conversion involves TE0 to TM0 polarization conversion mediated by an even-parity acoustic mode that can be excited by a conventional symmetric interdigital transducer. Co-localization of the optical and acoustic fields within a single resonator enhances their interaction, while the signal is separated from the strong optical pump by propagation direction, polarization, and frequency. Combining full-vector electromagnetic and acoustic calculations with a pulsed entanglement transfer protocol, we obtain a vacuum coupling of g0/2π = 2 kHz for the designed perimeter and show that cooperativity of C = 1 is achieved with a calculated on-chip pump power of approximately 2.94 μW for Qopt = 107 and Qac = 105. Under these assumptions, the protocol enables entanglement transfer from a superconducting qubit to a flying telecom photon, supporting the proposed architecture as a compact platform for integrated quantum transduction.

DOI
10.1063/5.0356014

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We present a numerical study of the design of a microwave-to-optical quantum converter based on Brillouin backscattering in a single thin-film lithium niobate racetrack resonator. The conversion involves TE0 to TM0 polarization conversion mediated by an even-parity acoustic mode that can be excited by a conventional symmetric interdigital transducer. Co-localization of the optical and acoustic fields within a single resonator enhances their interaction, while the signal is separated from the strong optical pump by propagation direction, polarization, and frequency. Combining full-vector electromagnetic and acoustic calculations with a pulsed entanglement transfer protocol, we obtain a vacuum coupling of g0/2π = 2 kHz for the designed perimeter and show that cooperativity of C = 1 is achieved with a calculated on-chip pump power of approximately 2.94 μW for Qopt = 107 and Qac = 105. Under these assumptions, the protocol enables entanglement transfer from a superconducting qubit to a flying telecom photon, supporting the proposed architecture as a compact platform for integrated quantum transduction.
Linking superconducting quantum devices to optical fibers via microwave-optical quantum transducers may enable large-scale quantum networks. For this application, transducers based on the Pockels electro-optic (EO) effect are promising for their direct conversion mechanism, high bandwidth, and potential for low-noise operation. However, previously demonstrated EO transducers require large optical pump power to overcome weak EO coupling and reach high efficiency. Here, we create an EO transducer in thin-film lithium niobate, a platform that provides low optical loss and strong EO coupling. We demonstrate on-chip transduction efficiencies of up to and of optical pump power. The transduction efficiency can be improved by further reducing the microwave resonator’s piezoelectric coupling to acoustic modes, increasing the optical resonator quality factor to previously demonstrated levels, and changing the electrode geometry for enhanced EO coupling. We expect that with further development, EO transducers in thin-film lithium niobate can achieve near-unity efficiency with low optical pump power.
Efficient and stable microwave-optical transduction is a key enabling technology for distributed superconducting quantum computing and heterogeneous quantum networks. Electro-optic transducers based on thin-film lithium niobate (TFLN) have shown strong promise, but demonstrations to date have been limited by various factors such as low frequency bias drift, low efficiency, fabrication complexity, and scalability. Here we demonstrate the first integrated electro-optic microwave-optical transducers realized in thin-film lithium tantalate (TFLT), a material platform offering Pockels nonlinearity comparable to TFLN together with improved bias stability and high-power handling. We fabricate superconducting microwave resonators coupled to tunable photonic-molecule optical resonators using wafer-scale deep ultraviolet lithography, offering high-throughput production of hundreds of devices per wafer. Across six devices we observe coherent bidirectional conversion between C-band optical photons and 4.9-5.5 GHz microwave photons, with measured on-chip efficiencies and inferred single-photon coupling rates g_0/2π ~ 1 kHz consistent with theory. Continuous operation over multiple days is achieved using a static bias field with minimal feedback, demonstrating a major operational advantage. We further characterize optical loss statistics, microwave resonator performance, and optically induced added noise under pulsed pumping, finding less than one added photon for 100 microsecond pulses at the highest measured efficiencies. These results establish TFLT as a scalable and robust electro-optic platform for future quantum interconnects and modular quantum processors.
We propose a novel scheme for efficient and low-noise microwave-to-optical quantum transduction based on cavity-enhanced Brillouin interaction between telecom photons and 10 gigahertz phonons on a lithium niobate-on-Sapphire chip. By employing a dual-microring cavity design and tailoring the geometry dispersion, triply-resonant and phase-matching conditions can be achieved for the backward Brillouin process, resulting in strong vacuum coupling strengths between the confined phononic and photonic modes. Our numerical analysis, taking into account the material anisotropy of lithium niobate and sapphire, predicts near-unity internal conversion efficiency with only sub-milliwatt pump power. The unsuspended all-solid-state architecture offers superior mechanical stability and robustness, overcoming a major challenge in piezo-optomechanical systems. Furthermore, the broadband nature of the Brillouin process enables multi-channel conversion, a key advantage over existing approaches. This versatile platform opens up exciting opportunities for scalable quantum processors with hybrid photonic-phononic-superconducting integrated circuits.
Quantum networks of superconducting qubits linked by optical channels could leverage both the quantum information processing capabilities of superconducting circuits and the long communication distances provided by optical photons. Such networks would require high efficiency, low noise, and wide bandwidth transducers between microwave and optical frequencies. Transducers based on the Pockels electro-optic (EO) effect are particularly promising in this application for their direct conversion mechanism and potential for strong performance. EO transducers could also be used for sensitive optical modulators and low-noise detection of microwave and millimeter-wave signals. This dissertation presents recent work to create cavity EO transducers in thin-film lithium niobate, an integrated photonics platform that provides low optical loss and strong EO coupling. I first describe the theory of cavity electro-optics and how it can be used to generate high-efficiency transduction between microwave and optical fields. An initial device is presented and characterized, demonstrating per-photon on-chip transduction efficiencies of up to (2.7 ± 0.3) × 10^−5. A key benefit of the device described here, which is based on photonic molecule modes, is the ability to use a static electro-optic bias to trim the transducer into resonance. However, we find the migration of free carriers in thin-film lithium niobate reduces the EO response of the device to static fields, making such trimming ineffective. This carrier migration is a key challenge for enabling the promise of low-power electro-optics provided by thin-film lithium niobate devices. I characterize the low-frequency electro-optic response, which suggests that conduction occurs on the etched surface of lithium niobate. I show how this conduction can be reduced – and low-frequency EO performance improved – by changing the electrode design and annealing the devices. Following this, I describe the design and initial characterization of an improved transducer device. Finally, I describe a scheme by which even relatively low-efficiency transducers could be used to generate remote entanglement using an optically heralding scheme. Demonstration of such a system appears possible with current devices, suggesting that small opticallymediated quantum networks of superconducting qubits may be feasible soon.
Coherent transduction of quantum states from the microwave to the optical domain can play a key role in quantum networking and distributed quantum computing. We present the design of a piezo-optomechanical device formed in a hybrid lithium niobate on silicon platform, that is suitable for microwave-to-optical quantum transduction. Our design is based on acoustic hybridization of an ultra-low mode volume piezoacoustic cavity with an optomechanical crystal cavity. The strong piezoelectric nature of lithium niobate allows us to mediate transduction via an acoustic mode which only minimally interacts with the lithium niobate, and is predominantly silicon-like, with very low electrical and acoustic loss. We estimate that this transducer can realize an intrinsic conversion efficiency of up to 35% with <0.5 added noise quanta when resonantly coupled to a superconducting transmon qubit and operated in pulsed mode at 10 kHz repetition rate. The performance improvement gained in such hybrid lithium niobate-silicon transducers make them suitable for heralded entanglement of qubits between superconducting quantum processors connected by optical fiber links.
Abstract Superconducting cavity electro-optics presents a promising route to coherently convert microwave and optical photons and distribute quantum entanglement between superconducting circuits over long-distance. Strong Pockels nonlinearity and high-performance optical cavity are the prerequisites for high conversion efficiency. Thin-film lithium niobate (TFLN) offers these desired characteristics. Despite significant recent progresses, only unidirectional conversion with efficiencies on the order of 10 −5 has been realized. In this article, we demonstrate the bidirectional electro-optic conversion in TFLN-superconductor hybrid system, with conversion efficiency improved by more than three orders of magnitude. Our air-clad device architecture boosts the sustainable intracavity pump power at cryogenic temperatures by suppressing the prominent photorefractive effect that limits cryogenic performance of TFLN, and reaches an efficiency of 1.02% (internal efficiency of 15.2%). This work firmly establishes the TFLN-superconductor hybrid EO system as a highly competitive transduction platform for future quantum network applications.